In a gas laser that uses direct current to excite a gaseous laser medium contained therein, an electrical discharge is produced due to a flow of direct electric current through the laser medium. High current densities associated with the electrical discharge create a hot ionized plasma. This high heat, along with the high current density, causes sputtering the tube wall and electrode material. Sputtering in a process by which atoms or molecules are ejected from the surface of a material by bombardment with energetic ions. Sputtering causes material to be eroded from the inner surface of the laser electrodes, introducing contaminants into the laser gas, and contaminating the optics in the vicinity of the cathode, resulting in excessive optical losses and decreased laser life. Problems due to sputtering tend to increase in severity with increases of current density in the inner tube.
To avoid contamination of the optics, external resonators have been used that enclose the gas lasing medium within a tube sealed with windows mounted at Brewster's angle at each end. The tube is supported in a structure that also supports two confronting mirrors disposed outside and on the longitudinal axis of the sealed tube. Thus, the mirrors are protected from sputtering processes within the tube, but at the expense of including an external support structure that adds to the size and complexity of the overall laser apparatus.
It is also known to isolate the cathode in a side-arm tube connected to the central bore of a laser tube so that the path of the electrical discharge does not pass near the optics. However, since the side-arm tube is typically of the same diameter as the central bore, the resulting structure is cumbersome and does not lend itself to the degree of miniaturization achieved by the invention.